Impacts of geometric modifications on infrared optical responses of metallic slit arrays

This work numerically investigates optical responses (absorptance, reflectance, and transmittance) of deep slits with five nanoscale slit profile variations at the transverse magnetic wave incidence by employing the rigorous coupled-wave analysis. For slits with attached features, their optical responses can be much different due to the modified cavity geometry and dangled structures, even at wavelengths between 3 and 15 microm. The shifts of cavity resonance excitation result in higher transmittance through narrower slits at specific wavelengths and resonance modes are confirmed with the electromagnetic fields. Opposite roles possibly played by features in increasing or decreasing absorptance are determined by the feature position and demonstrated by Poynting vectors. Correlations among all responses of a representative slit array, the angle of incidence, and the slit density are also comprehensively studied. When multiple slit types coexist in an array (complex slits), a wide-band transmittance or absorptance enhancement is feasible by merging spectral peaks contributed from each type of slits distinctively. Discrepancy among infrared optical responses of four selected slit combinations is explained while effects of slit density are also discussed.